1. Field of the Invention
The present invention relates to a novel process for the preparation of phosphates by a particular melting operation, as well as to the phosphates produced by such novel process. More especially, this invention relates to the preparation of phosphates by electromagnetic induction melting of precursors thereof.
2. Description of the Prior Art
It is known to this art that certain phosphates may be produced by melting certain precursors thereof. Thus, halophosphates, which are salts of the anion PO.sub.3 X.sup.2-, in which formula X is a halogen, may be prepared by melting.
These salts are typically prepared by melting precursors which may be, on the one hand, salts of orthophosphoric acid, pyrophosphoric acid or metaphosphoric acid and, on the other, halide salts, such as alkali metal or alkaline earth metal halides and, in particular, sodium fluoride.
In the same manner, it is also known to this art that phosphates of the polyphosphate type, which are polymers of orthophosphates or pyrophosphates, too may be produced by melting the precursors thereof. Among the polyphosphates which can be produced by melting, those of the following categories are illustrative:
(i) the tripolyphosphates, which are salts of tripolyphosphoric acid (of which only the salts are known) of the formula: H.sub.5 P.sub.3 O.sub.10 ; and
(ii) the metaphosphates, which are salts of metaphosphoric acids of the formula (HPO.sub.3).sub.n, in which n is an integer (when n is equal to 3 or 4, the acid or its salt is in the cyclic form and, when n is greater than or equal to 5, the acid or its salt is in the linear form).
The polyphosphates may be prepared by melting orthophosphates.
Under the action of heat, the orthophosphates condense until the desired polymer is produced. However, other precursors may also be used.
Thus, tripolyphosphates may be produced by melting pyrophosphates, and metaphosphates by melting pyrophosphates, tripolyphosphates or other metaphosphates.
In this case too, the phenomenon which takes place is one of condensation under the influence of heat (or thermocondensation). This thermocondensation is accompanied by a release of water vapor.
In general, the phosphates are salified with one or more metallic elements.
The amount and the nature of these metallic elements may result from the phosphate precursors described above, but may also result from other precursors such as inorganic salts, for example, the halide salts indicated above, or alkali metal or alkaline/earth metal salts.
To date, the preparation of phosphates by melting has been carried out using furnaces of the glass furnace type. However, these furnaces present a number of drawbacks.
Thus, these large-sized furnaces are provided with walls fabricated from bricks of refractory materials bonded together by a cement. The refractory material is typically zircon.
As a result of this, the phosphates produced using these furnaces are always polluted or contaminated with zirconium, which can render them of little value in certain fields of application, for example, in agrochemicals and in foodstuffs.
On the other hand, these furnaces do not present the advantage of being multipurpose. Thus, if such furnaces have been used to prepare, for example, a polyphosphate of a metallic element, the later use of the same furnace to prepare a polyphosphate of another metallic element results, again, in pollution. In effect, trace amounts of the metallic element from the first polyphosphate will remain present in the glass furnace. When the latter is used to prepare the other polyphosphate, these trace amounts will reduce its purity. Because of this fact, a single glass furnace is generally used, without disadvantage, only for the preparation of a single phosphate.
In addition, it has been found that, when such furnaces are used for the production of corrosive materials, such as the halophosphates and certain polyphosphates, they become corroded and perforated after a certain period of time of use. This perforation would essentially appear to be caused by an attack by said corrosive materials on the cement bonding the bricks of the furnace.
It will also be appreciated that glass furnaces require large amounts of energy, thus rendering the cost of the materials to be prepared relatively high, and that, because of their large dimensions, several hours, if not several days, are necessary before melting temperatures are attained.